Missed in commit f3478392 "docs: Move deprecation, build
and license info out of system/"

Signed-off-by: Mao Zhongyi <maozhongyi@cmss.chinamobile.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20210723065828.1336760-1-maozhongyi@cmss.chinamobile.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>